Alkali metal chlorate, and particularly sodium chlorate, is an important chemical in the cellulose industry, where it is used as a raw material in the production of chlorine dioxide, which is an important bleaching chemical for cellulose fibres. Alkali metal chlorate is produced by electrolysis of an electrolyte containing alkali metal chloride according to the overall formula:
MeCl+3 H.sub.2 O.fwdarw.MeClO.sub.3 +3 H.sub.2 (Me=alkali metal) The process is cyclic, where in a first step the chloride electrolyte is brought to an electrolyser for the formation of hypochlorite, whereupon the solution is brought further to reaction vessels for further reaction to chlorate. Subsequently, chlorate formed is separated by crystallization.
The content of impurities must be low in an electrolyte containing chlorate and chloride ions, if the electrolysis carried out in a chlorate cell shall be energy efficient. Thus, calcium, magnesium, sulphate and fluoride ions cause depositions on the cathodes and thereby a higher operating voltage and energy cost. Sulphates and fluorides are also known to damage the anodes, thereby increasing the operating voltage and/or decreasing the lifetime.
Sulphate ions can be removed by addition of calcium ions under alkaline conditions. At a high pH, the calcium sulphate precipitated will contain a substantial amount of chromium-containing compounds and will be difficult to separate and wash. Co-precipitation of chromium-containing compounds means a loss of a valuable chemical and an environmental problem if the sludge containing precipitated calcium sulphate is to be disposed of.
U.S. Pat. No. 4,636,376 discloses a process for removal of essentially chromium-free sulphate from a chromate and sulphate-containing chlorate liquor. The process includes mixing the chlorate liquor having a pH of between about 2.0 and about 6.0 with a calcium-containing material at ,a temperature and for a time sufficient to form a sulphate-containing precipitate predominantly of glauberite (Na.sub.2 Ca(SO.sub.4).sub.2) , and separating the glauberite from the chlorate liquor. Removal of sulphate as glauberite, means either a substantial loss of chlorate electrolyte with chromium-containing compounds contained in the precipitated glauberite or dissolution of glauberite in a subsequent washing step.
U.S. Pat. No. 5,176,801 discloses a process for removal of essentially chromium-free sulphate from a chromate and sulphate-containing chlorate liquor. The process comprises the steps of adjusting the pH of the chlorate liquor to within the range of from about 2.5 to about 4.5, mixing the pH-adjusted chlorate liquor with a calcium-containing material at a sufficient temperature and time to form a precipitate of gypsum (CaSO.sub.4 * 2 H.sub.2 O) , and separating the gypsum precipitated from the chlorate liquor. A pH of the chlorate liquor below about 4.5, reduces the product yield due to decomposition of chlorate produced. Furthermore, some of the compounds formed by the decomposition are flammable and/or toxic, e.g. chlorine, various chlorine oxides and chromyl chloride. At a low pH, the fluoride ions present in the electrolyte will be released as hydrogen fluoride. Also, the need for expensive pH control is pronounced since the buffering capacity at a pH below about 4.5 is very limited. Thus, various methods have been proposed to keep the concentration of sulphate in the chlorate process at an acceptable level. Common to these methods is either expensive controlling equipment or excessive loss of chlorate and/or chromium-containing compounds. Also, precipitation at a low pH requires addition of a substantial quantity of acid. Subsequently, a substantial quantity of alkali needs to be added to restore the pH or raise it further. Extensive addition of pH adjusting chemicals is expensive, introduces impurities in the electrolyte and constitutes a substantial risk of decomposition of chlorate due to an instantaneous and local drop in pH.